A ring-shaped hydraulic seal has a radially inner surface and a radially outer surface. Either or both of those surfaces can be formed as a sealing surface. For example, a piston ring typically has a radially outer sealing surface. The radially outer sealing surface is a dynamic sealing surface for engaging the inner surface of a cylinder in which the piston ring moves with a piston. A shaft seal typically has both radially inner and radially outer sealing surfaces. The radially inner sealing surface is a dynamic sealing surface for engaging a shaft which moves through the shaft seal. The radially outer sealing surface is a static sealing surface for engaging the wall of a cylinder in which the shaft moves.
It is known that a hydraulic sealing surface will provide the lowest leakage of hydraulic fluid if it is formed of an elastomer. It is also known that a hydraulic sealing surface will have a lower coefficient of friction if it is formed of plastic. However, plastics do not seal as well as elastomers. Accordingly, there is a trade-off between the ability of a seal to control leakage and to avoid friction. Sealing surfaces are therefore commonly formed with both elastomeric and plastic portions. When a hydraulic seal having both elastomeric and plastic components is designed, the geometries of those components are fixed when the forming tools, such as molds, are made. If the relative sizes of the elastomeric and plastic sealing surface portions are to be changed in a new design, new forming tools must be made.